1. Field of the Invention
The invention relates to an off-hook detector for use in a subscriber line interface circuit comprising a transmission bridge in one of whose branches an asymmetry may be introduced or not introduced depending on the value of a control signal, and a difference detector whose inputs are connected to the two terminals of the transmission bridge from where the said branches are connected to the central battery and whose output supplies a logic signal whose states indicate the opening or closing of the loop.
2. Description of the Prior Art
The off-hook detector is an element which detects the establishment or the interruption of the line direct current or transverse current. This transverse current may be disturbed by longitudinal parasitic currents induced on the line by power mains frequencies and which have the same direction at a given instant in the two wires of the line. When they flow through the transmission bridge, each longitudinal current is thus added to the transverse current in one of its branches and is subtracted in the other branch. If the bridge is symmetrical, that is to say if the two branches have the same resistances, the effects of these currents are cancelled and a difference detector connected to the transmission bridge terminals then produces a correct indication of the state of the loop; this detecting mode is of common usage in electronic telephone exchanges. But if an asymmetry is introduced in the bridge, the longitudinal currents cause the appearance at its terminals, superposed on the applied direct current, of an alternating voltage which is equal to the product of the current intensity and the difference of the resistance in the two branches of the bridge. In the event of a bridge which is considerably asymmetrical, the value of this alternating voltage reaches, and even exceeds, the value of the direct voltage at the terminals of the bridge, for example 30 Volts for a value of the central battery voltage equal to 48 Volts, which thus gives rise to inadvertent activation of the difference detector, which may then furnish a faulty indication about the state of the loop. An asymmetry might, for example, be introduced in the transmission bridge when the resistance value in one single branch is increased in order to reduce the power consumption in the line in the case of a "false call", that is to say when a subscriber keeps the handset too long from the hook before dialing or when he forgets to put the handset back on the apparatus at the end of the conversation; it would therefore be sufficient if the central exchange conveys, after a specified time interval, a control signal which causes an electromechanical or electronic interruptor connected to the terminals of a high-value additional resistor in one of the branches of the bridge to close. But the correct detection of the state of the loop by means of the prior art method of differential detection entails the necessity to incorporate in the same manner an additional resistor of the same value in the other bridge, which is a particularly costly solution.